The World Health Organization on Monday declared the birth defects linked to the Zika virus an international health emergency. Up to 4 million cases of the virus are estimated for the Americas in the next year, though whether it actually causes the birth defects is still unclear.

The tropical virus is spread by mosquitoes and scientists are investigating its link to an epidemic of birth defects in Brazil. The WHO had already warned that the virus will likely spread to every country in North and South America, except for Canada and Chile, where it is too cold.

The WHO has cautioned that while there was no definitive proof that the Zika virus is responsible for the birth defects, such as abnormally small heads, but Director-General Dr. Margaret Chan said last week that "the level of alarm is extremely high."

Since the first reported case of Zika infection triggered by Aedesaegypti mosquito bites in Brazil in May, the virus has rapidly spread spread to 21 countries and territories in the Americas, with mosquito-borne cases as far north as Puerto Rico.

"Zika virus will continue to spread and will likely reach all countries and territories of the region where Aedes mosquitoes are found," said the WHO statement.

In Brazil, where the mild flu-like virus has infected more than a million people, the disease has been linked to nearly 4,000 cases of severe brain birth defects in infants. Zika has also been linked to an earlier outbreak of Guillain-Barre syndrome, a rare paralytic disease, in French Polynesia.

"The circumstantial evidence is suggestive and extremely worrisome," WHO Chief Margaret Chan told the executive board of the world public health organization on Monday.

The U.S. Centers for Disease Prevention and Control has issued a travel advisory suggesting pregnant women avoid travel to 22 countries in Latin and South America. El Salvador, a largely Roman Catholic nation, has suggested that women avoid pregnancy there until 2018 to avoid the birth defect risk.

"There has been a near daily drumbeat of infections reported and unexpected health impacts," global health expert Davidson Hamer of Boston University told BuzzFeed News. "It really is an explosive spread, and looks very serious."

The Aedes mosquitoes that carry the Zika virus survive as far north as the United States' Gulf Coast and as far south as Argentina in the Western Hemisphere. Because people in the Americas have not been exposed to Zika before, they have no immunity to the disease, making its spread very likely.

In the U.S., the Aedes mosquito that carries Zika is widely found in South Florida and along the Gulf Coast. A related "Tiger" mosquito that might carry the disease travels as far north as the Great Lakes.

The WHO statement also notes one possible case of sexual transmission of Zika virus in the current outbreak, which is tied to an Asian strain of the disease that has evolved from the original African strain first described in Uganda in 1947.

"The possibility of sexual transmission really needs to be investigated," public health expert Daniel Lucey of Georgetown University told BuzzFeed News. "We know what we need to do to stop mosquitoes but we don't want to be surprised by another way to get infected."